This invention relates generally to ultrasonic testing systems and more particularly, to an ultrasonic wheel probe for rolling along a workpiece to be tested for flaws, having a plurality of ultrasonic acoustic transducers and an acoustic barrier immersed in a coupling fluid. The wheel probe is of the type having a container with a substantially annular outer surface for rotation about an axis of rotation, said wheel probe rolling on a workpiece to be tested, such as a length of pipe.
U.S. Pat. No. 4,615,218 which issued to the applicant herein on Oct. 7, 1986, describes an ultrasonic wheel probe comprised of an acoustic barrier interposed between transmitting and receiving transducers, so as to isolate the transmitting and receiving transducers from one another, thereby preventing crosstalk communication between the transducers which would otherwise occur through the coupling fluid. As described in the patent, the acoustic barrier of one embodiment is mounted on the yoke of the wheel probe so as to extend substantially to an annular inner surface of the container, thereby almost completely sealing the transducers from one another. The acoustic barrier is mounted on the yoke using a resilient, sliding arrangement which permits the distance between the axis of rotation of the container and the outermost extent of the acoustic barrier to be reduced. Thus if the contour of the workpiece being tested requires a deformation of the generally annular outer surface of the container or wheel, then the acoustic barrier can be retracted somewhat to accommodate for such deformation. The transmitting and receiving transducers can be angled within the container so as to permit focusing of the ultrasonic signal at a substantially predeterminable depth within the specimen being tested, the transmitted acoustic energy being propagated through the coupling fluid in a direction which is substantially parallel to, and on one side of, the major plane of the acoustic barrier; which transmitted acoustic energy enters the workpiece and is reflected back along the other side of the acoustic barrier to the receiving transducer. A positional transducer is provided which performs both transmitting and receiving functions and is arranged to transmit its acoustic energy directly to the back of the acoustic barrier, in a direction parallel, and substantially coincident with, the major plane of the acoustic barrier. Yhe reflected signal from the acoustic barrier to the positional transducer corresponds to the position of the acoustic barrier as it is retracted and extended in accordance with the deformation of the outer surface of the wheel probe container, thus monitoring the position of the acoustic barrier. Feedback triggering circuitry can be coupled to the positional transducer so as to define time periods during which echoes resulting from defects within the workpiece being tested can be expected to occur. This function can be performed by gating circuitry which is responsive to the feedback triggering circuits, and which can be arranged to eliminate any boundary reflections, thereby removing the need to gate out interference reflections which would otherwise reduce the material thickness that could be tested. Such a self-adjusting system allows critical segments of the workpiece to be tested with relative reliability. The teachings of U.S. Pat. No. 4,615,218 are specifically incorporated by reference herein.
As taught in the applicant's pending applications Ser. Nos. 735,334 filed on May 17, 1985 and 49,161 filed on May 13, 1987, the use of a plurality of transducer pairs in improved arrays can increase the effective footprint of the wheel probe with respect to the workpiece. As this footprint is increased, monitoring the actual distance of the barrier with respect to the surface of the workpiece, becomes more critical. The prior art wheel probe described in the U.S. Pat. No. 4,615,218 uses a transceiving positional transducer to detect the position of the acoustic barrier with respect to the workpiece being measured. The positional transducer transmits a signal to the acoustic barrier. This signal is reflected by a portion of the barrier and the reflected signal is received by the same transducer. From the time interval between transmission of the signal and detection of the echo signal, the position of the barrier and hence the distance from the transducer array to the surface of the workpiece is determined. As long as the footprint of the area being inspected remains small, this method of measuring transducer array to workpiece distance is adequate. However, as the inspection footprint increases due to, iner alia, the use, of multiple transducer pairs within the wheel probe, the topography of the workpiece becomes more critical. If a bump is encountered by the prior art wheel probe, the acoustic barrier shifts position accordingly thereby causing a momentary aberration in the transducer array to surface distance measurement, not representative of the true distance.
It is therefore the object of the present invention to provide a wheel probe with an acoustic barrier, which features an improved method of measuring the distance of the transducers to the surface of the workpiece.